“The best time to figure out your W-4 is right after you prepare your taxes,” said Nathan Rigney, lead tax research analyst at the Tax Institute at H&R Block.

“You need to keep in mind that we’re already in April, so you have some catching up to do,” he said.

There’s the catch: Using the IRS withholding calculator alone might not be enough to pinpoint the right amount you should withhold.

“A lot of people don’t understand the line items and you have to make sure you’re accounting for that accurately,” said certified financial planner Debbie J. Freeman, a CPA and director of financial planning at Peak Financial Advisors in Denver.

“Unless you understand taxes, it’s not something you should fully rely on,” she said.

Your W-4How much income tax your payroll company pulls from your paycheck depends on the number of allowances you claim on Form W-4.

The more allowances you take — you can claim them if you have dependents or if you itemize deductions on your taxes — then the less tax you will have withheld from your pay.

If you withhold too much tax, you’ll get a large refund from Uncle Sam next spring, but you’ve basically given the government an interest-free loan.

If you withhold too little, you’ll end up on the hook for taxes owed.

“If you can take a stab at it, it’s worthwhile to use the calculator, but I worry that people will fill it out wrong, fail to withhold enough and have a surprise next year,” said Chris Benson, a CPA and principal at L.K. Benson & Co. in Towson, Maryland.

Don’t forget state taxesThe biggest catch with the withholding calculator is that it doesn’t tell you how to adjust for state and local taxes.

After crunching the numbers for a New Jersey-based married couple that files jointly and with one dependent, the IRS calculator suggested that the higher-earning spouse claim zero allowances and the lower-earning spouse claim six.

However, that result doesn’t consider the Garden State’s individual income tax rate, which is as high as 10.75%.

“The W-4 is supposed to change your federal withholding, but often employers give you the same allowances on the federal and state side,” Freeman said.

Taking six allowances for both federal and state likely would have resulted in the couple owing New Jersey the following April, she said.

Instead, the lower-earning spouse could take three allowances on the federal return and three on the state, Freeman said.

Contact your employer’s payroll department to let them know how you would like to deduct for state taxes.

Work with your CPAUser error and unfamiliarity with the appropriate deductions and credits may also result in questionable calculator results.

Though you can plug in your data for the child and dependent care credit, the child tax credit and more, you have to be aware as to whether you qualify for those breaks.

People who itemize their deductions may take more allowances, thus reducing the amount of taxes pulled from their paychecks.

Further, if you have side gig income in addition to your W-2 income at work, you’ll need to revisit your withholding and ensure you’re paying enough taxes.

It doesn’t hurt to review your W-4 with your tax preparer and discuss potential changes, especially now that you’ve filed for 2018.

“The struggle with more precise withholding is that it’s ‘garbage in, garbage out,’” said H&R Block’s Rigney. “You need accurate inputs. It won’t be as simple as writing a number of allowances and calling it good.”